Rectifiers are often used to provide a DC power supply from an AC source. In such applications, the rectifier elements often comprise thyristors to provide a soft-start of the rectifier circuit by regulating the DC output voltage during the initial DC voltage ramp-up. Typically, the thyristors are triggered according to the firing angle as illustrated in FIG. 1A. As can be seen, the firing angle of the thyristors is a linear progression in time. In other words, a constant increment is added to the firing angle from each zero-crossing of the phase of the input voltage until the DC bus is fully charged. As a result, the DC bus voltage (across the capacitance of the DC output) rises with an s-curve profile (FIG. 1B) as would be understood by the skilled person. Correspondingly, the current in the DC bus rises to a high peak and then falls as illustrated in FIG. 1C.
As a result, the various support components of the DC power supply in question must be sized according to the large current peak. This means, for example, that fuses must be rated above the high peak current value which requires more costly components. This can result in input fuses being rated at three times the rated output current of the DC power supply. Further, the DC power supply as a whole is subjected to a higher level of strain owing to the more extreme current conditions that are present during the charging of the DC bus.
It is therefore desirable to provide a soft-start of a DC bus that reduces the peak current during charging of the bus to a lower level whereby fuses and other components of the system can be rated at a lower value.